Switching devices have been conventionally proposed that operate light emitting diodes (LEDs) (JP 2004-536434 A, hereafter referred to as “Document 1”).
The switching device disclosed in Document 1 is provided with a single-ended primary inductance converter (SEPIC).
Document 1 discloses three capacitors (a bypass capacitor, a capacitor and buffer capacitor), and a switching element, as electronic components that make up the SEPIC. Document 1 further discloses three inductors (a self-inductor, as well as a primary winding and a secondary winding of a transformer), and a diode, as electronic components that make up the SEPIC.
Improvements in power factor are desirable in lighting devices such as the switching device disclosed in Document 1.
To improve the power factor, the capacitances of the bypass capacitor and the capacitor in the switching device disclosed in Document 1 must be set to be comparatively small.
In this switching device, however, a concern arises in that, when the capacitances of the bypass capacitor and the capacitor are set to be comparatively small, the resonance current may flow in a closed loop circuit that includes the bypass capacitor, the self-inductor, the capacitor and the buffer capacitor. Accordingly, there is a possibility that the operation of the SEPIC becomes unstable when attempting to improve the power factor in the switching device disclosed in Document 1.
When the capacitances of the bypass capacitor and the capacitor are sufficiently large, i.e. when the resonance frequency of the closed loop circuit is sufficiently small with respect to the switching frequency of the switching element, a triangular-wave current flows in the diode, and the operation of the SEPIC is stable. When, by contrast, the capacitances of the bypass capacitor and of the capacitor are small, i.e. when the resonance frequency of the closed loop circuit is close to the switching frequency of the switching element, the current flowing in the diode has a resonance current superimposed onto the triangular-wave current flowing in the diode, and the operation of the SEPIC becomes unstable as a result.